The present invention relates to a novel method for color matching of a thermosetting powder coating composition, the method being capable of color matching with ease.
Conventionally, color matching of powder coating compositions have been made as by the following method to realize the color specified by the user. For example, a coloring pigment is mixed with and dispersed in a powder resin, a curing agent and the like, and the obtained dispersion is mixed to melt and disperse the coloring pigment, the curing agent and the like in the powder resin using, e.g., a twin-screw dispersing device, followed by cooling, coarse grinding, fine grinding and sieving, thereby giving the contemplated color-matched powder coating composition.
The above-mentioned preparation of color-matched powder coating compositions poses no problem in preparing a large amount of powder coating composition with a specified color. However, when powder coating compositions of various colors are prepared in small amounts, namely, in the case of small-lot manufacture of paints with different colors, numerous problems are entailed which include a great loss of powder coating compositions, considerable labor, intensified contamination and so on.
In order to overcome the above-mentioned problems, a variety of methods for color matching of powder coating compositions have been proposed. For instance, WO90/06345 discloses a method wherein two or more kinds of powder coating compositions with different colors are mixed together and granulated with an aqueous acrylic resin as a granulating agent to provide a color-matched powder coating composition. Japanese Unexamined Patent Publication No.188586/1995 describes a method wherein two or more kinds of powder coating compositions are dryblended for color matching, and the dryblend is granulated with a synthetic resin as a binder. However, these conventional methods necessitate the use of an aqueous acrylic resin and a resin binder both having a high molecular weight to prevent the degradation of film properties, resulting in irregular granules due to a high viscosity of the resin components and in decrease of thermal flowabillty of the granulated powder coating composition which leads to the impairment of film properties such as surface smoothness, specular reflectance and the like.
An object of the present invention is to provide a method for color matching of a powder coating composition, the method being capable of giving a powder coating composition which achieves good finished appearance without flooding and is excellent in film properties such as surface smoothness, specular reflectance, adhesion and the like, and the method being capable of facilitating color matching and small-lot manufacture of powder coating compositions with different colors.
Other objects and features of this invention will become apparent from the following description.
According to the present invention, there is provided a method for color matching of a thermosetting powder coating composition (hereinafter referred to as xe2x80x9ccolor matching method Ixe2x80x9d), the method comprising the steps of: (1) dryblending two or more kinds of starting colored thermosetting powder coating compositions of different colors having an average particle size of 20 xcexcm or less to achieve color matching, and (2) granulating the dryblend to a particle size in the range suitable for powder coating, by heating the dryblend with agitation to a temperature at which the dryblend melts at the particle surfaces but does not melt in the particle interiors.
According to the present invention, there is also provided a method for color matching of a thermosetting powder coating composition (hereinafter referred to as xe2x80x9ccolor matching method IIxe2x80x9d), the method comprising the steps of: (1) dryblending two or more kinds of starting colored thermosetting powder coating compositions of different colors having an average particle size of 20 xcexcm or less to achieve color matching, and (2) granulating the dryblend to a particle size in the range suitable for powder coating, by (i) adding a solid binder having an average particle size of 20 xcexcm or less and a lower melting point than the starting powder coating compositions and then (ii) heating the resulting mixture to a temperature at which the binder melts but the starting powder coating compositions do not melt.
According to the present invention, there is further provided a method for color matching of a thermosetting powder coating composition (hereinafter referred to as xe2x80x9ccolor matching method IIIxe2x80x9d), the method comprising the steps of: (1) dryblending two or more kinds of starting colored thermosetting powder coating compositions of different colors having an average particle size of 20 xcexcm or less with a solid binder having an average particle size of 20 xcexcm or less and a lower melting point than the starting powder coating compositions to achieve color matching, and (2) granulating the dryblend to a particle size in the range suitable for powder coating, by heating the dryblend to a temperature at which the binder melts but the starting powder coating compositions do not melt.
According to the present invention, there is further provided a method for color matching of a thermosetting powder coating composition (hereinafter referred to as xe2x80x9ccolor matching method IVxe2x80x9d), the method comprising the steps of: (1) dryblending two or more kinds of starting colored thermosetting powder coating compositions of different colors having an average particle size of 20 xcexcm or less to achieve color matching, and (2) granulating the dryblend to a particle size in the range suitable for powder coating, by (i) compressing the dryblend at a pressure sufficient to melt the particle surfaces of the dryblend for forming a sheet or grains, and then (ii) grinding and classifying the sheet or grains.
The present inventors conducted extensive research to overcome the foregoing problems of the prior art and found the following solution. The solution is a method comprising dryblending two or more kinds of starting thermosetting powder coating compositions of different colors with a specific particle size to achieve color matching, and granulating the dryblend to a particle size suitable for coating, by heating and agitating the dryblend, or by adding a specific solid binder, or by grinding and classifying a sheet or grains formed by compressing the dryblend. The method makes it easy to match the color of a coating composition and produces a colored thermosetting powder coating composition excellent in application characteristics, finished appearance and film properties. The present invention was accomplished based on these novel findings.
As used herein, the term xe2x80x9cgranulatexe2x80x9d is intended to mean xe2x80x9cto agglomerate powder particles into granules or agglomerates having a greater diameterxe2x80x9d.
Starting Colored Thermosetting Powder Coating Composition
Colored thermosetting powder coating compositions for use as the starting materials in the color matching method of the invention each comprise a coloring pigment and a resin component comprising a curing agent and a thermosetting powder resin which is melted and fluidized when heated. The thermosetting powder resin has a softening temperature of about 30 to about 100xc2x0 C.
The coloring pigment to be used herein is at least one pigment selected from white pigments, black pigments, red pigments, yellow pigments, blue pigments, green pigments, scaly aluminum powders, colored mica powders, copper powders, tin powders, and stainless steel powders. The amount of the coloring pigment to be used can be suitably selected to impart satisfactory film properties such as hiding power and surface smoothness to the film formed from the obtained powder coating composition. Although variable with the type of the pigment used, the amount of the pigment is about 0.1 to about 100 parts by weight, preferably about 1 to about 80 parts by weight, per 100 parts by weight of the powder resin component.
Examples of thermosetting powder resins include resins having a functional group which reacts with a curing agent when heated. Specific examples of such resins are acrylic resins, polyester resins, epoxy resins, fluorine-containing resins, silicone resins and hybrids of these resins. Suitable functional groups include hydroxyl, carboxyl, epoxy, blocked isocyanate, etc.
Curing agents useful herein include those having a functional group which reacts with the functional group in the powder resin to form a cured coating film and those capable of initiating polymerization of the functional group in the powder resin. Examples of curing agents for hydroxyl-containing powder resins include amino resins and blocked isocyanate compounds. Examples of curing agents for carboxyl-containing powder resins include polyepoxides and xcex2-hydroxyethyl alkylamides. Examples of curing agents for epoxy-containing powder resins include polycarboxylic acids, initiators for cationic polymerization of epoxy groups, etc. Examples of curing agents for blocked isocyanate-containing powder resins include polyols.
Examples of amino resins include hexamethoxymelamine resin, hexaethoxymelamine resin, xe2x80x9cCymel 303xe2x80x9d (tradename, a product of Mitsui Cytec. Co., Ltd.) and xe2x80x9cSumimal M-55xe2x80x9d (tradename, a product of Sumitomo Chemical Co., Ltd.). Examples of blocked isocyanate compounds include those prepared by blocking an aliphatic, alicyclic or aromatic polyisocyanate with a blocking agent such as a phenol, xcex5-caprolactam or alcohol. The aliphatic, alicyclic or aromatic polyisocyanate may be isophorone diisocyanate, hexamethylene diisocyanate, xylylene diisocyanate, tolylene diisocyanate, hydrogenated xylylene diisocyanate, hydrogenated tolylene diisocyanate or the like. Examples of polyepoxides include homopolymers and copolymers of triglycidyl isocyanurate or glycidyl (meth)acrylate, and xe2x80x9cCeloxide 2021xe2x80x9d (tradename) and xe2x80x9cEHPE-3150xe2x80x9d (tradename) both manufactured by Daicel Industries, Co., Ltd. Examples of xcex2-hydroxyethyl alkylamides include xcex2-hydroxyethyl propylamide. Examples of polycarboxylic acids include adipic acid, sebacic acid, suberic acid, succinic acid, glutaric acid, maleic acid, fumaric acid, dodecanedioic acid, pimelic acid, azelaic acid, itaconic acid, citraconic acid and like aliphatic polycarboxylic acids and their anhydrides; terephthalic acid, isophthalic acid, phthalic acid, trimellitic acid, pyromellitic acid and like aromatic polycarboxylic acids and their anhydrides; and hexahydrophthalic acid, hexahydroisophthalic acid, methylhexahydrophthalic acid and like alicyclic polycarboxylic acids and their anhydrides. Examples of polyols include trimethylol-propane, sorbitol and tris(2-hydroxyethyl)isocyanurate. Examples of initiators for cationic polymerization of epoxy groups include benzyl-4-hydroxyphenyl methylsulfonium hexafluoroantimonate and benzyl-4-hydroxyphenyl methylsulfonium hexafluorophosphate.
The starting colored thermosetting powder coating compositions may contain, in addition to the powder resin, the curing agent and the coloring pigment, curing catalysts, fillers, UV stabilizers, UV absorbers, fluidity modifiers, anti-cissing agents, oil repellents, antibacterial agents, rust preventives, etc., where necessary.
The starting powder coating compositions can be prepared by conventional methods such as the melting and kneading method and lyophilizing method. The melting and kneading method comprises dryblending a powder resin, a curing agent, a coloring pigment and optionally other components in a mixer or the like, and melting and kneading the dryblend with heating, followed by cooling, coarse grinding, fine grinding and sieving. The lyophilizing method comprises dissolving or dispersing a powder resin, a curing agent, a coloring pigment and optionally other components in an organic solvent having a melting point of xe2x88x9240xc2x0 C. or above, freezing the solution or dispersion usually at 10 to xe2x88x9240xc2x0 C., and removing the solvent under reduced pressure for drying. Useful organic solvents include tert-butanol and dioxane.
According to the present invention, the starting powder coating compositions have an average particle size of 20 xcexcm or less, preferably 1 to 10 xcexcm, more preferably 1 to 8 xcexcm. If the average particle size exceeds 20 xcexcm, the granulated powder coating composition is given a large average particle size, thereby reducing the electrostatic coating workability and degrading the surface smoothness and the flooding resistance of the film.
In the present invention, the average particle size of the starting powder coating compositions and granulated powder coating composition is expressed in terms of a particle size with a cumulative frequency of 50%, as calculated from the results of particle size distribution analysis. The particle size distribution is analyzed using, e.g., xe2x80x9cMicrotracxe2x80x9d (tradename for an FRA particle size analyzer manufactured by Nikkiso K.K.).
Color Matching Method I
The color matching method I of the invention comprises the steps of: (1) dryblending two or more kinds of starting colored thermosetting powder coating compositions of different colors having an average particle size of 20 xcexcm or less to achieve color matching, and (2) granulating the dryblend to a particle size in the range suitable for coating, by heating the dryblend to a temperature at which the dryblend melts at the particle surfaces but does not melt in the particle interiors.
(1) Color Matching Step
In the color matching method I, color matching is first conducted by dryblending two or more kinds of starting colored thermosetting powder coating compositions of different colors having an average particle size of 20 xcexcm or less.
The color matching step is executed, for example, by dryblending two or more kinds of powder coating compositions of different colors in a high-speed mixer or the like to give a uniform mixture and inspecting the color of the mixture, whereby a dryblend of the desired color is obtained.
There is no limitation on the temperature and time involved in dryblending. Usually it is suitable to carry out the dryblending at about 5 to about 40xc2x0 C., preferably about 8 to about 30xc2x0 C. for about 0.5 minute to 24 hours, preferably about 1 minute to 1 hour. Further, for example, in the case of using a high-speed mixer, the mixture is agitated by an agitator revolving at about 100 to 5,000 rpm, preferably about 200 and 2,000 rpm and a chopper revolving at about 1,000 to 10,000 rpm, preferably about 2,000 to 6,000 rpm.
Optionally, relations between the mixing ratio of two or more kinds of starting powder coating compositions and the color of the resulting film may be previously input into a computer, so that the mixing ratio of two or more kinds of powder coating compositions which gives a film of the desired color can be automatically determined. In other words, the color matching step can be automated and can be more conveniently conducted by utilizing a computerized color matching method.
(2) Granulation Step
In the color matching method I, the dryblend of powder coating compositions having the desired color given in the color matching step is heated with agitation to a temperature at which the dryblend melts at the particle surfaces but does not melt in the particle interiors, to granulate the dryblend to a particle size in the range suitable for coating. Thus, the contemplated color-matched thermosetting powder coating composition is obtained.
The granulation step is performed by mixing and dispersing the dryblend with heating at a temperature at which the particles of the dryblend melt at the surfaces and adhere to each other but are not fused together or markedly deformed.
The temperature conditions for granulation vary with the softening temperature and melting viscosity of the starting coating compositions, the production amount and other factors, and thus suitable conditions are selected according to the starting powder coating compositions used and other factors. Generally, the granulation is conducted at a temperature of about 40 to 80xc2x00 C., preferably about 40 to 60xc2x0 C. for about 1 minute to 20 hours, preferably about 5 minutes to 10 hours.
Color Matching Method II
The color matching method II comprises the steps of (1) dryblending two or more kinds of starting colored thermosetting powder coating compositions of different colors having an average particle size of 20 xcexcm or less to achieve color matching, and (2) granulating the dryblend to a particle size in the range suitable for coating, by (i) adding a solid binder having an average particle size of 20 xcexcm or less and a lower melting point than the starting powder coating compositions and (ii) heating the resulting mixture to a temperature at which the binder melts but the starting powder coating compositions do not melt.
(1) Color Matching Step
The color matching step in the color matching method II can be carried out in the same manner as in the color matching method I.
(2) Granulation Step
In the color matching method II, the solid binder is added to the dryblend obtained in the color matching step, and the mixture is agitated and dispersed with heating at a temperature at which the binder melts but the starting powder coating compositions do not melt, whereby the melted binder binds the particles of the starting powder coating compositions to each other to granulate the dryblend to a particle size in the range suitable for coating. As the result, the contemplated color-matched thermosetting powder coating composition is obtained.
The temperature conditions for granulation vary with the softening temperature, melting point and melting viscosity of the starting coating compositions and the binder, the production amount and other factors. Thus, suitable conditions are selected according to the starting powder coating compositions used and other factors. Generally, the granulation is conducted at about 20 to 80xc2x0 C., preferably about 30 to 60xc2x0 C. for about 1 minute to 20 hours, preferably about 5 minutes to 10 hours.
Preferred as the solid binder is at least one binder having a lower melting point than the starting powder coating compositions and selected from the group consisting of resins, curing agents for the starting powder coating compositions, higher alcohols, monobasic acids and waxes. Examples of resins include acrylic resins, polyester resins, epoxy resins and fluorocarbon resins. Examples of curing agents include polycarboxylic acids, carboxylic anhydrides, blocked isocyanate compounds, polyol compounds and amino resins. Examples of higher alcohols include cetyl alcohol and stearyl alcohol. Examples of monobasic acids include lauric acid, myristic acid, palmitic acid, stearic acid and behenic acid. Examples of waxes include polyethylene wax, paraffin wax, carnauba wax, microcrystalline wax and montan wax. It is usually preferable that the binder have a melting point of about 10 to 70xc2x0 C., particularly about 20 to 60xc2x0 C.
The amount of the solid binder to be added varies with the melting temperature, binding properties and viscosity of the binder, and other factors. Thus, a suitable amount is selected according to the binder and starting powder coating compositions used. It is usually suitable to add the binder in an amount of about 0.1 to 50 wt. %, preferably about 3 to 30 wt. %, relative to the starting powder coating compositions.
The solid binder can be dryblended with the starting powder coating compositions after grinding the binder to an average particle size of 20 xcexcm or less, preferably about 1 to 10 xcexcm in a jet mill or like apparatus.
Color Matching Method III
The color matching method III comprises the steps of: (1) dryblending two or more kinds of starting colored thermosetting powder coating compositions of different colors having an average particle size of 20 xcexcm or less with a solid binder having an average particle size of 20 xcexcm or less and a lower melting point than the starting powder coating compositions to achieve color matching, and (2) granulating the dryblend to a particle size in the range suitable for coating, by heating the dryblend to a temperature at which the binder melts but the starting powder coating compositions do not melt.
(1) Color Matching Step
The color matching step in the color matching method III can be performed in the same manner as in the color matching method I, except that the solid binder is dryblended with the two or more kinds of starting colored powder coating compositions. Alternatively, the binder may be blended with the components of the starting powder coating compositions when preparing said compositions, and melted, kneaded and ground together with the components.
Specific examples, average particle size and amount to be added of useful binders are the same as in the color matching method II.
(2) Granulation Step
In the color matching method III, the dryblend obtained in the color matching step is mixed and dispersed with heating at a temperature at which the starting powder coating compositions do not melt but the solid binder melts to bind the particles of the starting powder coating compositions to each other, thereby granulating the dryblend to a particle size in the range suitable for coating. Thus, the contemplated color-matched thermosetting powder coating composition is obtained.
The temperature conditions for granulation are the same as in the color matching method II.
Color Matching Method IV
The color matching method IV comprises the steps of: (1) dryblending two or more kinds of starting colored thermosetting powder coating compositions of different colors having an average particle size of 20 xcexcm or less to achieve color matching, and (2) granulating the dryblend to a particle size in the range suitable for powder coating, by (i) compressing the dryblend at a pressure sufficient to melt the particle surfaces of the dryblend for forming a sheet or grains, and then (ii) grinding and classifying the sheet or grains.
(1) Color Matching Step
The color matching step in the color matching method IV can be performed in the same manner as in the color matching method I.
(2) Granulation Step
In the color matching method IV, the dryblend obtained in the color matching step is compressed at a pressure sufficient to melt the particle surfaces of the dryblend for forming a sheet or grains, and the sheet or grains are ground and classified to a particle size in the range suitable for powder coating. Thus, the contemplated color-matched thermosetting powder coating composition is obtained.
The pressure to be applied varies with the softening temperature and melting viscosity of the starting powder coating compositions, the production amount and other factors. Thus, a suitable pressure is selected according to the starting powder coating compositions used and other factors. It is generally suitable to apply a pressure of about 100 to 10,000 Kg/cm2, preferably about 500 to 4,000 Kg/cm2. If the pressure applied is less than 100 Kg/cm2, the particles of the starting compositions do not sufficiently adhere to each other, so that the sheet or grains, when being ground, separate into the particle size of the starting compositions. On the other hand, if a pressure over 10,000 Kg/cm2 is applied, the particles of the starting compositions are fused together to form a solid product which does not consist of granules but has uniform structure. As the result, small particles that have not been granulated will be present in the obtained powder coating composition, reducing the application workability.
In the color matching method IV, a compression apparatus is used which comprises a transfer unit for microfine powders and a compression unit. Examples of such apparatus include xe2x80x9cRoller Compactor RCP-200Hxe2x80x9d (tradename) and xe2x80x9cRoller Compactor MRCP-200xe2x80x9d (tradename), both manufactured by Kurimoto, Ltd.
The compressed product is finely ground using a pin disc or like means, and sieved through a suitable sieve for classification, whereby the contemplated granulated powder coating composition suitable for powder coating is obtained.
In the granulation step, at least one of the aqueous binders, solid binders, aqueous organic solvents, aqueous surfactant solutions and the like may be used for promoting granulation.
The particles of the thus obtained granulated powder coating composition of the invention are agglomerates each made of several particles of the starting compositions. It is preferable that, in the agglomerates, the particles of the starting compositions substantially retain their shape before granulation, and adhere to each other so as not to separate into particles during transfer from a paint tank to an electrostatic coating apparatus or during electrostatic spray coating.
The color-matched granulated powder coating compositions obtained by the methods I to IV have an average particle size in the range suitable for powder coating, i.e., generally about 10 to about 50 xcexcm, preferably about 12 to about 25 xcexcm. If the average particle size is below the range, the coating composition tends to deposit with a lower efficiency when applied by electrostatic coating operation, whereas above the range, the resulting coating film tends to be given a lower surface smoothness. Therefore, an average particle size outside the foregoing range is undesirable.
The color-matched powder coating composition obtained by the present method is applied to a substrate and baked to form a cured coating film.
The substrate may be any of conventional substrates which can be used in powder coating. Examples of useful substrates include metals, surface-treated metals, plastics, these substrates coated with a coating composition, etc. The powder coating operation can be carried out by conventional methods, for example, electrostatic powder coating methods, frictionally electrified powder coating methods, fluidization dip coating methods, etc. There is no limitation on the film thickness. However, it is suitable that the obtained film have a thickness of about 20 to 200 xcexcm, preferably about 20 to 150 xcexcm, more preferably about 20 to 80 xcexcm, when cured. The deposited composition is baked at a temperature of about 100 to 250xc2x0 C., preferably about 160 to 200xc2x0 C. for about 3 to 120 minutes, preferably about 20 to 40 minutes.
The color-matched powder coating composition obtained by the present method can be used, without limitation, in applications in which conventional powder coating compositions are used, such as automobiles, electric appliances, furniture made of steel, office goods, construction materials, pipes, etc.
The present invention will be described in detail with reference to the following Examples and Comparative Examples in which all parts and percentages are by weight. The present invention is not limited at all to these examples.
The starting powder coating compositions used in the Examples and Comparative Examples are as follows:
(1) xe2x80x9cEverclad #4800 Whitexe2x80x9d: tradename of Kansai Paint Co., Ltd., a blocked isocyanate-curable polyester resin powder coating composition with an average particle size of about 5 xcexcm and a melting point of about 82xc2x0 C. (as determined by differential scanning calorimetry), which is white in color and forms a coating film with a lightness value L* of about 95 in L*a*b* color specification system (JIS Z 8729); and
(2) xe2x80x9cEverclad #4800 Grayxe2x80x9d: tradename of Kansai Paint Co., Ltd., a blocked isocyanate-curable polyester resin powder coating composition with an average particle size of about 5 xcexcm and a melting point of about 85xc2x0 C. (as determined by differential scanning calorimetry), which is gray in color and forms a coating film with a lightness value L* of about 70 in L*a*b* color specification system (JIS Z 8729).